Within recent years, imitation coffee creamers, milks, whipped toppings, cheese, sour creams, etc. have increasingly been accepted by consumers as a replacement for more costly natural dairy products. Milk proteins are most commonly used in these imitation dairy products because of their exceptional flavor and functional properties. The functional attributes of milk proteins in natural and synthetic cheese products are unique. These unique functional attributes play not only a vital role in the finished imitation dairy products, but also in their preparation.
The art has desired to replace milk protein with vegetable proteins. Unfortunately vegetable proteins do not possess the necessary prerequisital properties to function as a caseinate replacement in these imitation dairy products. Vegetable proteins would be a highly desirable imitation dairy product component, provided an economical and effective means for simulating the unique efficacy of casein could be found.
Vegetable proteins (including soy) are sensitive to a wide variety of conventional food processing and recipe conditions which do not normally affect milk proteins. Relatively mild physical processing conditions such as heating, drying, moisture level, etc. are known to adversely affect its properties. Factors such as the nature and character of the recipe additives, concentrations, ionic strength, pH, temperature and recipe preparation affect protein interreactions and its functional properties. Vegetable protein molecules undergo complex association, disassociation and chemical reactions with one another as well as other molecules which may be present in any given system. To compensate for such functional deficiencies supplemental non-dairy additives such as food stabilizers, gums, etc. are typically required. Such additives often result in more costly imitation products of an inferior quality. Consequently, vegetable protein isolates are usually relied upon primarily as a nutritional protein source in such imitation products instead of as a functional component.
An article by C. V. Morr (Jr. of Amer. Oil Chem. Soc., March 1979, page 383) reports that the functional and chemical properties of vegetable proteins are complex. Sedimentation by centrifugation studies are conventionally used to identify the different indigenous types of soy proteins. These centrifugal sediments are commonly referred to as the 2S, 7S, 11S and 15S fractions, which respectively corresponds to peak molecular weights of approximately 25,000; 160,000; 350,000 and 600,000. An illustrative native soybean seed analysis (on a weight basis) will typically yield approximately 7% 2S, 34% 7S, 42% 11S and 9% 15S. The major soy protein components are the 7S and 11S fractions. Factors such as seed type, climatic and growing conditions, as well as isolate processing conditions can alter the molecular weight distribution and the relative proportions of these protein fractions. The larger molecular weight fractions are comprised of a plurality of subunits which are known to undergo ionic association and disassociation. This contributes to the quaternary structure and complexity of the soy proteins when used in aqueous systems. These subunit interreactions significantly contribute or interfere with the functionality of soy properties in food recipes.
Soy proteins which gel upon heating have been reported. U.S. Pat. No. 3,870,801 by Tombs discloses a mesophase defined as "fluid aqueous composition, capable of being heat-coagulated, containing from 15 to 50% dissolved, undenatured plant protein and sufficient water-soluble salts to keep the protein dissolved and having a pH in the range of about 4 to 6." The mesophase is prepared from high NSI soy flakes by extracting the soluble constituents at a low temperature in the presence of excess water and with a small amount of sodium sulfite followed by the removal of insoluble carbohydrate material therefrom by centrifugation (pH 4.6-4.9), isolation of the protein from the supernatant and reconstitution of the isolate in an aqueous salt solution with special precautions being taken to avoid oxidative polysulfide formation. The mesophase is reportedly heat-coagulable at 90.degree. C. and is useful as a protein binder in meat applications.
U.S. Pat. No. 4,188,399 by Shemer also discloses a heat-coagulable viscous soy protein product. According to the Shemer patent a high NSI soybean flour is subjected to aqueous extraction at a pH 5.1-5.9 in the presence of sodium sulfite at a low extraction temperature to extract soluble proteins and cabohydrates therefrom. The liquid protein is then adjusted to a pH 4.5 with phosphoric acid to provide a viscous fluid containing more than 70% of the 7S fraction. The viscous fluid material of Shemer is disclosed as a heat-coagulable binder for synthetic and natural meat applications.
British Patent Specification No. 1,377,392 discloses a dry, substantially undenatured salt-containing soy protein composition. The soy protein isolate "entails precipitation of the isolate from an aqueous extraction prepared from defatted soy meal in the presence of a water-soluble sulphite, bisulphite, or dithionite salt, preferably an alkali metal (including ammonium) salt . . . ." According to the British patentees, the protein isolate is then reconstituted in an aqueous salt solution to form a liquid composition containing from 0-50% dissolved soy protein and spray-dried to provide a free-flowing pale cream powder which is reportedly useful in preparing foodstuffs such as soy protein extrudates. The spray-dried powder is described as readily reconstituted in water and set by heat at temperatures ranging from 80.degree.-150.degree. C.
A U.S. Patent by Melnychyn et al. (U.S. Pat. No. 3,630,753) discloses a process for producing a freeze-dried soy protein isolate. It is obtained by an alkaline extraction (e.g. pH 8.5) of the protein and water-soluble components in the presence of a specific type of oxidizing or thiol bearing reagents which are capable of reacting with disulfide linkages. The extraction is preferably conducted at about 170.degree. F. which will result in partial hydrolysis of the protein. The crude extract is then clarified by centrifugation, the protein precipitated at pH 4.5 and 100.degree. F. followed by its recovery by centrifugation, washing, its redissolving in water at pH 7.0, the freezing and lyophilizing thereof to obtain a dry soy protein isolate powder. The isolate is reportedly suitable in formulating liquid foods such as imitation milk and infant feeding formulations.
Additional references reporting upon the gelation properties of soy protein include Puski (Cereal Chem. 52:665-664 (1975)); Circle et al. (Cereal Chem. 41:157-172 (1964)); Catsimpoolas et al. (Cereal Chem. 47:559-570 (1970)); U.S. Pat. Nos. 3,741,771 by Pour-El et al.; 2,561,333 by Beckel et al.; 3,870,812 by Hayes et al. and 2,495,706 by DeVoss et al. Further references disclosing the affect of reducing agents upon protein fractions include Briggs et al. (Archieves of Biochemistry and Biophysics 72:127-144 (1957)); Nash et al. (Cereal Chem. 44:183-192 (1967)) and Wolf (Jr. Agr. Food Chem. 18: No. 6, 969-976 (1970)).
Recognizing a long-felt need, the inventors have discovered that vegetable proteins can be effectively converted into an isolate product form which will permit its usage in imitation dairy products. These unique vegetable proteins may be obtained under preparatory conditions which are believed to restructure the native protein constituents into a unique, high-NSI, low viscosity producing isolate product. Although high NSI soy isolate hydrolyzates are known, commercial grades of soy isolate products which have not been subjected to enzymatic or chemical hydrolysis typically have a water solubility ranging between about 20 to about 70 NSI (nitrogen solubility index). Substantially unhydrolyzed soy isolates of an NSI of 100 are now possible. Unlike conventional isolates which fail to possess the characteristics of milk caseinates the subject isolates have been found to be unexpectedly useful as a partial or complete replacement for milk caseinates. The water-solubility, bland flavor, ability to form clear, tender, elastic, heat-induced gels (as opposed to brittle, non-elastic or rigid gels), water-absorption, fat emulsification, tolerance to salt and other dairy product additives uniquely distinguish these isolates from conventional isolates. The composite properties of these unique vegetable protein isolates render them useful for applications heretofore deemed impossible with conventional vegetable protein isolates.